Loading system



PatentedV `L?, 193e()` PATENT vfori-1.cl3* 1!" irrmorny" nl ysr'rim, or nn'rinnrennj NEW JERSEY; ``Ass` 1ci1\ro1,` To BELL, TELEPHONE -LABoRAToRrEsg)INcoitrortn'rEnforj`NEW YORK, N; Y;, 'A CORPORATION or Nnvv This invention relatesto loading systems for telephone lines and theflike,v and has for lan object to improve :the impedance characteristics of loaded lines.l Y. t

It is well known that the ordinary loaded line-has an impedance characteristic whicliis,

a practically pure resistancebntwhichvaries 4considerably with fre uency. This variation is particularlyyinarke in the case of loaded cable circuits. On theother hand the impedance ofunloaded open wirelines 1s practicallytindependent "of frequency so that whncaiiunloaded @Pen wire of line is joined to an ordinary .laded'line considerable ref lection losses are introdiiced` In order to, avoid such losses and Valsofto'simplfy the irn- Apedance balancing; i arrangementsat repeat-V ers it is desirable thatthe loaded line should have an impedance `whichVis resistive and practically constant with frequency.

In' one specific embodiment this invention comprises an-openl wireline or cable periodically loaded means of loadi nnits 'comprising inductancefcoilsand ,con ensers connectedin parallel with eachother in each side of the line. These loadingv units maybe distributed along the lineat approximately tlie Sfarnel distances apartfas'lare thefllo'adin coils in linesloaded by the"` Pupinfmetho Ina modifiedvform,uthis` invention prises acompositefiloaded vsystemuin `which Nhenterniinatediinid-coil; such a system has a characteristic impedance whichis a sub?" stantially constantresistance throughout the transmission rangefff 9 the sectionv'or sections at eachterminal are loaded Vaccording"to the-method of this illvention` `as-abovej described i the intermediate sections are loadedy according to the Y ordinary-metliod,iieiythePiipin method,` It

is, PSSbl'tQ 11S@hscmbiheii because for frequencies the transmissionenge e@ I 'ne 'loaded 'rthflcadng units efhisnife'llton, is substantially eqaltothe inidsec:` toneharacterstic iinpedance"ofv a linelcaded in; theiisuall manner,

. This.imfeniioegiiwiii be beati @ideama by referencetofthefollowing detailed. de

" `I'LoAmNo SYSTEM 1927, Serin Na. 198,459.l

scription taken in connection -withg the e ac-` companying drawlng 1n which; Y

1 vhows a network snbstantiallyiV equivalent` to `a section of acable'or line in which the distributed Series resistance and shnt capacity ofthe line are represented by equivalenty impedances;

Fig. 2i1l1istrates the equivalent network of; Fig`.1 combined with the u Sllaltype'offload-- infrcoils and terminated mid-coil; 'f

'i Cigy shows'schematically a portion i withr the loading transmission line loaded units of this invention; l t d Figrfl shows a network equivalent toa sin-f` gle,l lnid-coil terminated section ofV the l present invention; and g 6 [hows schematicallya composite typeof loaded, linesot `this inventions l "It e sletion ofcable crepe, wire line which the series resistance. `and shunt Capacity are the controlling factors, be represented by y the equivalent T networkshown in. Fg-V 1. where R- is thetotalf seriesiresistance; ofthe section and S and fC are respectively Lthe length ofy the eablesection andthe ,Capacity for eaclilnnit oflen th,so thattlieprodnct SC "ifsthe totalcapa'city, such a section loaded inthe usiial. manner inay be represented lby' tlienetwork shown in Fig.` 2, where Lv'isvjthe` totahinductanceof theloading coil. vIn this 'Case asis well known, the Cut-off frequency gf thelcded Sectin is pproxmatelyggive fe-M/gf@ t ,e thel ilnpedance-.fifqeeiei characteristics of cables loaded .in the ordi- Vnary'manner and in accordancel vviththe 70 i sov full coil bein installed at the end of the first complete loa ing section. The network of Fig. 2 represents a mid-coil terminated section. section respectively the characteristic impedances in the transmission range are given approximately by impedance is approximately a pure resistance, which varies with the frequencies as a function of the ratio of the frequency to the cut-off frequenc l Referring to lifig. 5 'curve B representsthe im nce-,frequency characteristic of a cab e loaded in the ordinary manner (Pupin method) with mid-coil termination, and as shown, the impedance decreases with increasing'frequency, .approaching zero at the cutolf frequency which for this case is about 2800 cycles per second. Curve A shows the impatience-frequency characteristic for the same type of cable with mid-section termination, and, as shown, in this case the impedance increases rwith increasing frequency, ap-V l proaching infinity at the cut-oil" frequency.

Curve C, is the impedance-frequency charac-V teristic of a non-loaded openwire line of approximately thesame nominal impedance. t is therefore obvious that if the ordinary type of loading is employed and the loaded cable having either termination is oined directly to a non-loaded open wire line, large reflections will occur at the junction due to this difference in the impedances. y

Fig. 3, vshows a line or cable loaded in accordance with the method of this invention in which anti-resonant networks, each of which consists of an inductance coil connected in parallel with a condenser, replace the ordinary type of loading coils.y Fig. 4 shows a T type network equivalent to a single midcoil terminated section of the system of Fig. 3,. This type of loading unit, like the usual type, has a low pass characteristic, and its cut-off frequency is approximately ffm/m .(4)

Its nominal impedance is the same as for lines loaded 1n the usual manner f APN/g, 5)

and is independent of the capacity of the seriecondcnsers. The Characteristic impedi- For cables loaded mid-coil and midi tion, throughout the frequency band is unaltered, this, as hereinafter explained, is advanf tageous ratherthan otherwise since it allows a. cable ofcomposite loading to be formed. The characteristic impedance of a cable terminated at mid-load (mid-coil) Hwith the anti-resonant type of loading is, however, affected by the series condensers and 1s approximately wherefoo', the anti-resonantfrequency of the loading network, is

ExceptY where this anti-resonant frequency is very close in value to the cut-off frequency, the effect will be to make the characteristic impedance very uniform throughout the transmission band. l (Comparing Formulas (6) and (2), it is seen that for the two types of loading the mid-load impedance differs by the factor and that this factor may be varied over a wide range by a suitable choice of foo, that is, by a suitable choiceof capacity for the condensers.

`In Fig.` 5 curve D shows the im edancefrequency'characteristic for a loade line of the type shown inFig. 3 in which the anti-reso# nantfrequency is 1.25 times the cutoi frequcncy,t at is, Y

For a fixed value of loadin coil inductance in each anti-resonantnetwor and fixed spacing of such networks, the cut-off frequency is somewhat lower than for the ordinary type of loading, as can be seen b' comparing Equation (4) with Equation l), the amount of reduction of the kcut-off frequen depend'm on thechoice of anti-,resonant equency o the loading networks. Throughout the effective transmission range fthe anti-resonant type of loading gives attenuation and phase characteristics which are nearly' the same as for the ordinarytype of loading, except at frequencies near the cut-off frequency. The difference kin transmission characteristics may be roughly expressed bysaying that the Cut-.off frequency .is somewhat reduced as Yau misses compared with the erdinary type loading, ai-

though. this effect may be minimizedor pre` vented by reducing the'lo'adin spacing.

,food- A attenuation I andY phase characteristics..

Thisis particularly `true'when toll entrance cable .orI 'submarine V"cable must joined'to open Wire lines without causing refiections at the junction points".V For example, in acase 'in which an entrancel cable is used tovfconnect a repeater yto an open vvire line, the repeater having a cut-offV frequency :of about 2600 cycles per second and thefnominal cutoif1fre-- quency of the cable being fixed at 7200 cycles per second only 36% ofthe transmission ranffe is used.` ,Thev impedance ofthe cable loaded in the usualfmanner, asl shown by curves A and B in Fig. 5l will stillvbe considerably different from thatv of the open `Wire line, as shown by curve C. However, if the anti-resonant networks in accordance with the invention are substituted for the ordi.

nary type of loading, andthe same ,values of loading coil inductance and the 'samefspacingy are chosen as in the case ofk ordinary loading' 540() cycles and the impedance yof the cable would be'practically the saine as that ofthe vopen yWire line throughout theentire eii'ective transmission range. Also the attenua-` tion and phase shiftA characteristics would be practically identical with those ofthe cable loaded in the ordinary manner. f

Fig'. 6 shows a composite type' of loaded line in which some ofthe sections are loaded `vvitli anti-resonantnetworks and some are loaded in the ordinary manner. This arrangement y is possible because the mid-sectioncharacter# istic impedance with either type of loading may be made very closely the same by choos ingthe same cut-olf frequencies and nominal impedances. y The particular advantagev of thistype of composite loaded-line is that the uniform impedance cham'cteristicsy 'are desired primarily "at the ends lof theV cable span,v thatvis, at repeater and terminal points By4 loading a few sections at either* end of the-- ing' interfering frequencies induced from extraneou's sources, andinajsmuch as the canti! resonant net-Work has an attenuation charac# teri'stic which rises sharply yabove the cut-ofi frequency, as in the case of the corresponding typeoflow pass filter, this proposedtype of loading may be of considerable value from this standpoint. Furthermore, the sections of this typek when inserted in the cable, may

have different" frequencies of anti-resonance,

sothat the attenuation is maintained high for some distance above the cut-off frequencyl .lnloading phantom circuits and side Icir,` cuits in j phantomed systems With the antiresonant type of loading network thecondensers forming a part of the loading network inthe side circuit pro'duce'no eect as far'as the phantom circuit is concerned, `and similarly those used in the rplizimtom circuit produce no effect on the side circuits. l

1j. A "ave transmission system comprising a plurality of sectionsV of'line, inductive means 'for periodically'loadin one of said sections in accordance with the upinsystem, and loading networks each comprisingin'-v ductance and capacity elements connected "in parallel with one another for periodically loadinganother of said'sections' Yfor substantially the samerange of frequ'enciesl 2. AWave transmission system comprising intermediate andrterminal sections of line, inductive means for periodically,loadingsaid intermediate 'section according'fto the Pupiln method, and loading 'networkseach comprisf ing-inductance and capacity elements co'n-` nected'in parallel with each `otherfor pe riodically loading'one of said terminal sections for substantially the sanie'range'of frequencies. Y ,I l

3. A Wave transmission system according* to the next preceding claim and a loadinglnet- Work for4 terminating said. terminal section and comprising inductance and capacity ele-` ments connected nparallel with each other and each having van impedance which is; a fraction of the impedancel of the correspond ing element of' said other loading networks.

4. A Wave transmission system comprising intermediate vand terminal sections of line,-

inductive meansV for periodically loading said* intermediate section accordingto' the' Pupin method, 'and loading networks' each comprising inductance and capacity elements ifo connectedin parallelY with each other forpe'- ricdicll'y leading one v01E said terminal sections for substantially' thesarne rangeof frequencies, said elements being so proper@ tioned asto givesaid 'terminal section substantially the sa'mecut-oi freq'uencyfl as said intermediated section and 4the Aimpe'dance' of the elements of the terminating network being substantially one-half the impedance' of the correspondingl elements of the other net'- works.`

A method silencing@ transmissie line,

for waves of a plurality of frequencies, which consists in inserting at regular intervals along the line and in series therewith, networks comprising inductance and capacitance in parallel wit-h each other, and assigning to said networks such values as to cause said networks to become anti-resonant at a frequency which is substantially greater than loading cut-off frequency and greater than any frequency to be transmitted by said line, whereby the line is givena substantially constant impedance throughout a large portion ofthe transmission frequency range.-

6. A method of loading a transmission line for waves of a plurality of frequencies, which consists of inserting at regular intervals alongthe line and in series therewith, networks comprising inductance and capacitance in parallel with each other, and assigning to said networks-such values as to make said networks anti-resonant at a frequency which is substantially 25% greater than the loading cut-off frequency, whereby the line is given a substantially constant impedance throughout a large portion of the transmission frequency range.

7. A loading system' comprising line-conductors and loading units connected-in series with each of said conductors for loading said conductors for waves of a plurality of frequencies, each unit including an inductance and a capacity element of such values as to cause the units to'become anti-resonant at a frequency which i is substantially greater than the loading cut-off frequency and greater than any frequency to betransmitted over said line conductors whereby the line is given a substantially constant impedance throughout a large portion of the transmission frequency range. y

8. A loading system comprising a pair of line conductors and loading units connected in series with each of said conductors for loading said conductors for waves of a plurality of frequencies, said units being-disposed at regular intervals along the conductors, each unit including an inductance andra capacity element of such values as to cause the units toibecome anti-resonant at aA frequency which is substantially greater than the loading cut-off frequency and greater than any frequency to be transmitted over said line conductors, whereby the line is given a substantially constant impedance throughout a large portion of the transmission range. f Y r 9. A wave transmission system comprising a line, a plurality of loading networks periodically distributed` along said line for loadingsaid line for waves of a plurality of frequencies, each of said networks comprising an inductance element and a capacity element connected in parallel with each other, and assigned such values as to same fraction of the impedance of the corresponding elements of the other loadin networks, whereby the yline is given a su antially-constant impedance throughout a large portion of the transmission frequency range.

10. A wave transmission system according to the next preceding claim in which the impedance* of each element of the terminating loading network is substantially one-half of the impedance of the corresponding elements of the other loading networks.

11. A loading system arranged in sections, each of which comprises line conductors and loading units `connected in series with said conductors, each unit including an inductance and a capacity element of such values as to give each section a maximum value of attenuation at a frequency above the cut-olf frequency and a substantial amount of `attenuation for all frequencies higher than the frequency of maximum attenuation throughout a wide range of frequencies.

12. A loading system comprising line conductors and networks for loading said conductors fora band of telephone frequencies, connected in series with each of said conductors, each network including an inductance and a capacity element of such values as to cause the networks to become antiresonant at a frequency which is substantially greater than the loading cut-off frequency and above the telephone frequency range, `whereby the line is given a substantially constant impedance throughout a large portion of said band.

13. A loading system comprising a transmission line and recurrent networks inserted at regular intervals along said line and in series therewith to load said line for a plurality of frequencies-to be transmitted, said networks comprising the sole loading elements in said line, each network comprising elements including an inductance and a capacitance in parallelwith eac-h other of such values as to cause the network to become antiresonant at a frequency which is substantially greater than the loading cut-off frequency, each network having the same combination of elements and with the same values as every other network in the line, whereby the line is given a substantially constant impedance throughout a large portion of the transmis-` sion range. Y

In witness whereof, I hereunto subscribe my name this 10th day of June, A. D. 1927.

TIMOTHY E. SHEA. 

